LED-based (retrofit) lamps are used more and more in home buildings and offices. Besides their high efficiency they attract consumers also due to new design features, different color temperatures, diming ability etc.
If LED lamps are connected to already installed/existing dimmers they need to be compatible to these dimmers, i.e. they need to cope with the high oscillations generated by the dimmer during the phase edges/cuts and to guarantee a minimum holding current during the whole cycle of the (phase cut) mains voltage. Finally, as a function of the conducting angle the LED lamp needs to reduce its optical output as a result of a lower operation current.
Currently, about 90% of installed phase cut dimmers worldwide are of leading edge or trailing edge types. These dimmers cut a part of the mains voltage, either at the beginning of the sinusoidal voltage (leading edge) or at the end of the sinusoidal voltage (trailing edge) and thus reduce the time energy is fed to the connected driver electronics.
In order to guarantee a proper operation, a minimum holding current of the electronic switch in the dimmer (which is typically a triac) needs to be drawn by the connected electronics through the whole cycle of the supplied mains voltage, even for the lowest dimming settings. However, at various times modern LED driving electronics needs only a low amount of current. For example a non-constant current may be drawn by the electronic circuit, for example with a higher current drawn at the beginning of the mains cycle and less during the remaining time of this cycle. Low wattage LED packages of course need a low current. If the efficiency of higher power LED packages increases further, then even high power LED packages may have low current requirements. US20120319621A1 and other prior arts disclose various solutions to make the dimmer happy: namely meet the dimmer's requirement on the minimum latch current. Their basic solution is when the load current is too small to keep the dimmer operating, an extra current is drawn from the dimmer to make the dimmer output current above its minimum latch current.
Usually at a dimmed state the LED converter (which consists of a driver circuit and a ballast circuit) delivers too much power to the LEDs, resulting in still rather high light outputs, even at small conducting angles of a phase cut dimmer. In order to compensate for this, surplus power delivered by the converter needs to be converted, typically to heat, by means of so-called bleeder constructions.
The applicant has proposed (but not yet published at the priority date of this application) a bleeder approach which comprises a circuit located on the LED board (a so-called level 2 bleeder), where sufficient heat spreading possibilities are available since the LEDs run at a lower wattage during dimming.
The aim of the bleeder construction is to allow deeper dimming levels. The proposed system draws a constant current when the bleeder function is active, so that the sum of LED current and bleed current stays constant.